The present invention relates to a method of reducing the loss of mineral fractions in the treatment of broke in paper production. The invention relates in particular to the treatment of the so-called broke from a paper machine, which is reusable raw material, so that as much of the mineral fractions the broke contains may be returned to the process and used as raw material for paper as efficiently and economically as possible.
The term broke means unsaleable paper, which usually may be returned to the paper manufacturing process to be used as raw material. The volume of broke from a paper machine is usually about 5-40% of the production of the machine; thus the share is quite remarkable. Broke comes up in different forms and various amounts of it are produced at different locations in the paper machine: at the wire section in edge trimming, when the web is broken, at the coater and the slitter in trimming the edges, as surface and bottom broke of reels and as broke reels. The main function of the broke system is to disperse the broke pulp coming from the paper machine and to return it to the process.
Generally the broke from a paper machine is dispersed in pulpers as required and the broke pulp flows from the pulpers are guided to a broke chest. The pulp is guided from the broke chest via consistency adjustment to the mixing tank of the paper machine.
A little more advanced treatment of broke typically comprises dispersing, storing, thickening, screening and defiberizing/deflaking of the broke and dosing it back to the paper production for use as raw material. One application is the following approach. Broke paper is dispersed in machine pulpers to pumpable form (consistency 4.0-5.5%). As the pulp dispersed in the pulper contains non-defiberized paper flocs the broke pulp must still be defiberized after storing before it is recycled to the process. Part of the paper machine broke (for example trimmings from the wire) is returned to the broke system at a low consistency. For this reason, the broke system must include concentration of the broke in order to prevent the storage capacity required from increasing unnecessarily high.
Broke is dispersed in pulpers disposed at the paper machine and at the finishing treatment. Paper pulp coming from different pulpers is usually collected to a wire pit disposed under the wire section from which the broke is transported via a broke tower to a pressure screen, which separates fiber bundles and impurities from the pulp. Reject from the pressure screen is guided to a vibrating screen and accept from this is guided back to the broke tower. Reject from the vibrating screen is discharged from the system. The fraction, which has passed through the pressure screen in the broke line, is pumped to a broke concentrator, which raises the consistency of the broke to the level of 4% required by the dosing system. The concentrated pulp is transported from the broke concentrator to a dosing tank from which the broke pulp is pumped via defiberizers to the mixing tank of the paper machine.
The system and method of treating broke described above is the same irrespective of the place where the broke is produced; thus, problems are caused by the different kinds of broke. Present broke treatment systems face problems mostly with coated broke. Paper machines producing coated grades require a separate treatment system for coated broke or at least a separate storing for dosing the broke at the proportion desired. The ash content (the term used for minerals and fillers) of coated broke is higher than the ash content of base paper and thus, if dosed unevenly, it causes fluctuations in the filler content of the base paper. Coated paper is also more difficult to disperse and therefore it often requires a treatment of its own which discharges the non-dispersed fraction from the pulp and returns it to the dispersion treatment. Further, the coating layer of coated broke is not dispersed fine enough by the present broke treatment but remains partly non-dispersed as flakes which, being large fractions, are rejected later in the process, i.e. in the hydrocyclones of the short circulation. The composition of this rejected pigment fraction would allow utilizing it as raw material for paper, but its particle size is too coarse.
The same applies also to recycled pulp systems which use coated magazine paper and other richly coated recycled pulp as raw material for recycled pulp. In recycled pulp dispersing systems, the coating pigment of the coated paper detaches from the fiber layer of the paper itself more or less as sheet-like fractions, which are partly dispersed by the process. The dispersion is, however, not complete and thus these non-dispersed coating pigment particles are discharged from the process as reject from the hydrocyclone cleaning in the screen room. However, most of the particles rejected have a raw material composition, which would allow using them in paper production as filler; filler may have to be added to the fiber suspension at a later stage of the paper manufacture. The size of these particles, however, causes problems at the paper machine itself if the particles are not disintegrated and thus made acceptable for use as filler.
An improvement in the prior art technology was provided by an approach disclosed in Finnish patent application no. 960351 according to which the filler/mineral fraction loss discharged as mill reject from hydrocyclone cleaning is reduced by treating the fraction, the mineral fraction content of which has been concentrated, in connection with the hydrocyclone plant in order to disintegrate the mineral fractions and to return them to the process. The method and the apparatus of the invention provide for example the following advantages: losses of fillers/minerals, water, chemicals, heat and fibers reduce; only the unusable fraction and the fraction, which cannot be converted usable, is discharged from the process in a highly concentrated form; the system is a continuous part of the short circulation or the screening system of a paper machine; in other words, the process conditions are constant and the operation free from disturbances. Further, the process is self-adjusting. For example, if the volume of the coarse fraction introduced into the system increases the system recycles to the process only the fraction it is capable of disintegrating and the rest is discharged from the system.
Thus, present methods employ treatment of the solid material rejected from the hydrocyclone cleaning plant of a paper machine short circulation. The prior art has, however, a few drawbacks. Particularly at paper machines producing large volumes of reject, such as machines manufacturing base paper for coating and/or machines using coated broke, the fractions rejected in the hydrocyclone cleaning originate from the coated broke. These rejected fractions are mainly non-disintegrated coating layer, i.e. coating flakes. The share in the pulp of fractions rejected during the hydrocyclone cleaning is quite high and this loads and wears out the hydrocyclone apparatus. As the volume rejected in the hydrocyclone cleaning is high, the solids losses become high despite the use of the treatment apparatus and cause raw material, transport, dump etc. costs in addition to the wearing of the apparatus. Further, the ash content of coated broke is higher than that of base paper which, with uneven dosing, results in fluctuations in the filler content of the base paper. Also, in the present approaches, which treat mineral fractions, pulps from different sources, such as paper machine broke, recycled paper and broke from the pulp section, are combined and treated in the same hydrocyclone plant. Then, the pulps having different filler contents are mixed with each other and the entire pulp volume must be treated in order to separate the coarse fractions.
A problem essentially influenced by the present invention is the operation and wearing of the short circulation apparatus. Another problem solved by the present invention is connected with the fluctuations in the filler content of base paper caused by the dosing of coated broke. The approach according to the invention essentially reduces the amount of filler-containing reject rejected in a hydrocyclone plant and to be transported to a dump. Also the volume of pulp to be treated to separate the mineral fraction reduces remarkably.
A preferred embodiment of the present invention is based on treatment of coated broke so that pigment flakes are separated/fractionated from dispersed coated broke to a separate treatment and thus disintegrated before the broke is returned to the paper machine mixing tank. In this way, the amount of the fraction rejected in the hydrocyclone cleaning reduces essentially because the pulp does not contain rejectable coating flakes. At the same time, the ash content of the pulp is more accurately controllable as the mineral fraction has been removed from the broke before the broke is guided to the paper machine mixing tank.
Pigment and mineral material in this application means all pulp coating materials. When mineral fraction is referred to it means coating flakes requiring more efficient treatment in order to be disintegrated.
The method of the present invention provides for example the following advantages:
Solids losses in the hydrocyclone cleaning are essentially reduced as the pulp does not contain anymore as much large coating flakes from coated broke as before.
The load of the hydrocyclone cleaning is reduced as the share of coarse rejectable fractions in the pulp is reduced. Then the hydrocyclone cleaning works better and more efficiently.
The runnability of the hydrocyclones is improved and the risk of clogging is reduced.
Wearing of the short circulation apparatus is reduced.
When the concentration of the pigment from the coated broke is high the pigment may be stored, transferred or even transported to another mill. In this way all the raw material may be utilized and no solid material usable as raw material need be transported to a dump.
After separation of the fiber fraction and the coating fraction, or fractionating, more attention can be focused on the treatment of the fiber part of the coated broke and the fiber properties of the broke may be optimized.
Wearing of the apparatus used for treatment of fiber, such as defibrators, is reduced as the mineral fraction has been separated/removed.
The treatment is advantageous as the volume of the pulp to be treated is remarkably reduced when, in order to disintegrate the mineral fraction, only the coated broke, or pulp having essentially a high content of coarse mineral fraction, need be treated and not all the pulp treated in the short circulation and coming to the paper machine.
The investment costs of the system are low. The invention is technically carried out with known commercial apparatus and the repayment period is short.
The system is easy to carry out, i.e. it is easily added to existing broke treatment systems.
The characteristic features of the invention become apparent in the enclosed patent claims.